1. Field of the Invention (Technical Field)
The present invention relates to optical inspection of microelectronic devices, in particular measurement of line profile asymmetry using scatterometry.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
The fabrication of a microelectronic device is a complicated procedure that uses a variety of equipment for the different process steps involved. First, the lithography process transfers the image being made into a light sensitive material known as photoresist. This image in photoresist, in turn, acts as a mask for the next patterning process known as etching. Etching is the process by which the resist image is transferred into a suitable material such as poly-silicon. Then the etched material is over-filled with some insulating materials, planarized if necessary, and the whole process begins again.
Throughout the entire process the devices being made should be symmetric in nature from step to step, i.e., a transistor gate if manufactured correctly will have equal left and right sidewalls as well as other features such as, but not limited to, equal left and right corner rounding. If errors occur during the processing this desired symmetry may be compromised, and as a result the device integrity or functionality may also be compromised. If the asymmetry is quite severe the device may not function at all.
The present invention is of a manner of performing symmetry/asymmetry measurements via scatterometry. Scatterometry is an optical inspection technique well suited for the measurement of symmetry or asymmetry on microelectronic devices. By analyzing the light scattered from an array of microelectronic features, measurements of the line profile can be made. In particular, a scatterometer that measures at complementary angles, i.e., +45 degrees from a position perpendicular to the surface and −45 degrees, is ideally suited for symmetry/asymmetry measurements because the reflectance properties of the line profile can vary at these angles, although complementary angles are not necessarily needed to detect asymmetry. To enhance the sensitivity of this effect the array of features should be placed in a particular orientation, known throughout the specification and claims as a general conical configuration, namely one in which the wave vector of the illuminating beam does not remain parallel to the array's plane of symmetry.
Prior art techniques typically employ “classic” scattering. These are measurements geared towards the measurement of surface roughness, defects, pitting, etc. However, the present invention is based on the physics of diffraction, with the measurements in the invention always occurring with respect to periodic features (such as line/space gratings).
Prior work in scatterometry used the technique for the measurement of line profiles in resist and etched materials. C. J. Raymond, et al., “Resist and etched line profile characterization using scatterometry,” Integrated Circuit Metrology, Inspection and Process Control XI, Proc. SPIE 3050 (1997). The present invention provides a technique for the measurement of asymmetric line profiles (e.g., unequal sidewall angles).
U.S. Pat. No. 5,963,329 discloses an analysis method used to obtain a scatterometry measurement. The comparison between the diffraction model and the measured signature data is typically done by pre-computing a series of modeled signatures and storing them in a library. Then, the measured signature is compared to the library of modeled signatures to determine the parameters of interest. The patent essentially discloses a real-time model regression, where the signature iterations are performed in real-time and the model is changed repeatedly until a satisfactory match is obtained. However, there is no disclosure of using such a technique to determine asymmetry. Note that the profiles in FIG. 2 and FIG. 4 are symmetric, and column 7, line 40, reads “. . . and to define the separation between the symmetric left and right edge profiles . . .”
U.S. Pat. No. 6,292,265 is concerned with measuring dishing, erosion and residue by way of monitoring the thickness of materials that are prone to these effects. Note column 3, line 15, which reads: “It is a major feature of the present invention to provide such a method . . . to prevent residues, dishing and erosion effects.” Note also column 3, paragraph 25, which reads: “The main idea of the present invention is based on . . . an optical monitoring system capable of thickness measurements in patterned structures.” The disclosure concerns CMP, which is a thickness/relief measurement, instead of the measurement of a full line profile and any asymmetries that may be present.
U.S. Pat. No. 5,912,741 pertains to measurements of surface microstructure/roughness and is based on classical scattering. The present invention is based on measuring a specific diffraction order—this patent is based on measuring diffuse reflectance or specular reflectance and not diffraction. There is also no mention of asymmetry measurements—this is because this measurement is not applied to line profile measurements.
U.S. Pat. Nos. 4,978,862 and 4,933,567 are also geared towards microstructure or microdefects in materials and not for physical line profile measurements. Crystalline defects and impurities that could be measured are discussed. There is no discussion about the measurement of diffraction intensity and how it could be used for the measurement of asymmetric line profiles.
U.S. Pat. No. 6,292,259 also relates to classical scattering measurements, such as that induced by pits and particles on the surface of a material. The novelty of this patent is that it can distinguish pits from particles, but it does not concern diffraction or diffraction intensity measurements.
U.S. Pat. No. 5,889,593 discloses an optical design that is capable of an angular dependent measurement. The patent discloses an optical imaging array (reference 60 in the figures). It does not discuss the importance of using complementary angles for the measurement of line profile asymmetry. In fact, the repeated line structures shown in FIG. 6 are symmetric.
U.S. Pat. No. 5,905,573 relates to the perturbation of an evanescent field that is formed in a waveguide/resonator. The local presence of topography on a material being examined causes a perturbation of the probing radiation and hence a ‘blip’ in the intensity of the probing radiation. This measurement is not based on the physics of diffraction. It is also not an angle-dependent intensity measurement—if the power of the radiation in the resonator increases or decreases, then there must be some topography present that is perturbing the field. Furthermore, there is no discussion of asymmetry measurements.
U.S. Pat. No. 5,982,489 relates to depth measurement only and not line profile (so it is not possible to determine line profile asymmetry). It is also based on an interference technique as opposed to a diffraction/scattering measurement.
U.S. Pat. No. 5,864,394 also relates to defect measurements. Column 1, line 35, reads: “. . . for inspecting anomalies (contaminant particles and pattern defects) on surfaces.” There is no mention of diffraction measurements and hence no discussion of the measurement of line profiles, much less any asymmetry that might be present.
U.S. Pat, No. 5,637,873 pertains to an optical design with applications to the emissivity/reflectance of surfaces and coatings. This is not a patent with applications to patterned features or the measurement of the profiles of those patterned features.
U.S. Pat. Nos. 5,313,542, 5,475,617 and 5,640,246 pertain mainly to an optical design that allows for the measurement of partial or full hemispherically scattered light. The applications of this design are to measure light scatter caused by scratches, blemishes, bubbles, subsurface defects, and surface roughness, and are classic light scattering applications. There is no mention of measuring diffraction for the purposes of line profile measurements, and no discussion of asymmetry.
An NMRC Scientific Report regarding Photonics Research from 1999 is also related strictly to classic scattering applications (i.e., the measurement of surface defects). While it does involve an angular scattering measurement, it does not measure diffraction and hence is not applied for line profile measurements.
P. Ding, et al., “Light Scattering Study of Roughness and Dishing on Post-CMP Wafers” (date unknown) is also geared towards classic scattering. The measurement itself is an angular scattering measurement and does involve the measurement of diffraction (the samples are periodic lines and spaces). However, the measurement data is geared towards roughness and dishing applications, and does not involve a comparison to a theoretical diffraction model. It therefore does not measure the line profiles and hence no mention of asymmetry measurements is made.
“2pi Steradian Detection of Pits” (date unknown) relates to sub-surface defects and this literature is thus related to classic scattering applications. The hardware employed is an angle resolved scatterometer, but unless such hardware is coupled with a diffraction model and an analysis method, the measurement of line profile asymmetry cannot be performed.